Corona devices are known which are used in reproduction machines employing a photoconductive element to produce copies of documents to be reproduced. During reproduction processes such as xerography, it is necessary to apply a uniform level of charge to a photoconductive surface such as photoreceptor member, which charge will subsequently be selectively dissipated by exposure to light as part of the operation of the reproduction machine. In xerographic processes, the non-discharged portions retain their charge in the form of a latent image on the photoconductive surface, and when subsequently brought into contact with toner material, will retain toner on the surface of the photoreceptor in the areas where the charge has not been dissipated. In a commonly used charging device, a high voltage generally in the range of 5000 to 8000 volts is applied to a wire extending between insulating end blocks mounted within a channel or shield and held closely adjacent a surface to be charged to create a corona spray which imparts electrostatic charge to the surface to be charged. In another similar device, (referred to hereinafter as a scorotron) providing more uniform charging and preventing over charging, two or more wires are provided with a screen or control grid held at a uniform lower potential than the wires disposed between the wires and the photoreceptor. This arrangement suppresses the electric field between the photoconductive surface and the wires, and reduces ion current flow to the photoreceptor. In yet another variation of a charging device commonly used with photoreceptor elements requiring negative charging, a dicorotron comprising a wire coated with a relatively thick dielectric material such as glass in an arrangement otherwise similar to a corotron may be used. Negative precharging of certain photoreceptor types prior to charging to a uniform positive potential is often desirable as well.
Difficulties are observed when using corona charging devices that produce a negative corona. While not clearly understood, it is believed that various nitrogen oxide species are produced by the corona, and that these nitrogen oxide species are adsorbed by solid surfaces, particularly the conductive members which form part of the corona producing device. After exposure to the nitrogen oxide species, when the machine is turned off for extended periods of time, it is believed that the nitrogen oxide species are gradually desorbed either in the same form as originally noted, or in other forms. Whatever the nature of the process, when operation of the machine is resumed, a copy quality defect is observed in the copies produced where a line image deletion or lower density image is formed across the width of the photoreceptor at the portion of the surface thereof which was at rest adjacent to the corona producing device during the period of idleness. While the exact mechanism is not clearly understood, it is believed that the desorbed species interact with the photoreceptor material layers increasing the lateral conductivity thereof, so that the photoreceptor cannot retain a charge in image fashion to be subsequently developed with toner. This causes narrow line images to blur, wash out or not be developed as a toner image. This defect has been observed in a variety of photoreceptor and negative charging arrangements as well as negative pre-charge arrangements.
Prolonged exposure of the photoreceptor to the desorbed species appears to have the effect of increasing the defect. The problem is noted after relatively short operational periods, and subsequent periods of idleness. Cleaning the photoreceptor after initial exposure to the desorbed species with a cleaning solution such as alcohol as a healing effect as the initial reactions appear to be only at the photoreceptor surface. However, after prolonged exposure, the reaction tends to penetrate into the photoreceptor and cannot be cleaned away. The defect is reversible to some extent by a rest period, but the period required is on the order of several days. Frequent cleaning and extended periods of nonuse are undesirable remedies.
A variety of solutions have been proposed for the prevention of the problem of what is commonly referred to as photoreceptor deletion, primarily directed to coating or plating conductive members with non-reactive materials. In one solution, the shield portions of corotrons were plated with relatively non-reactive materials such as gold. It is believed that gold provides a relatively inert surface which does not adsorb the nitrogen oxides species. While gold has a positive effect in reducing photoreceptor deletion, the expense is undesirably high. Coatings, such as lead, reactive metal based paints, or alkali metal silicate coatings, which are intended to absorb and/or neutralize the nitrogen oxide species have also been used with some success, but still add substantially to the cost of the corona generating members. Additionally, while somewhat successfully preventing photoreceptor deletion, the alkali metal silicate coatings produce byproducts of the reaction in the form of a powder. When used in conjunction with scorotron charging, the powder, presumably an alkali metal nitrate, collects on the control grid, effects the operational characteristics of the device, alters the electrical characteristics of the screen, and causes non-uniformities in the electric field between the photoreceptor and the corona charging wires. Coating members to avoid adsorption appears to be effective, but adds significant cost to the manufacture of corona devices.
It is known that circulation of air through the area adjacent the corona charging member and the photoreceptor appears to have a preventative effect on the problems associated with nitrogen oxide species. Removal of corona by-products by circulation of air appears to prevent, to some degree, the adsorption of the nitrogenous species by the conductive elements of the corona producing device, and accordingly, their subsequent release. Thus for example, in U.S. Defensive Publication No. T940,022 by Rodda, an electrostatographic copying system is shown provided with a blower and filter system, which through circulation of air, has the advantage of exhausting and filtering corona byproduct species to prevent photoreceptor image deletions.
It is also known in corona devices that the corona emissions produced thereby are associated with corona winds comprised of ionized air molecules which acquire significant velocities such that their momentum carries the ions towards the surface to be charged. If the flow thereof is substantially unidirectional, these winds create a vacuum effect behind the charge member which draws air from that area towards the charge receiving surface. U.S. Pat. No. 3,324,291 to Hudson suggests a corotron arrangement producing a corona wind substantial enough to be useful in cleaning the surfaces of a charging device of particulate matter such as toner particles.
In a variation of known charging devices, an electrically conductive electrode strip having projections, teeth, scalloped portions, or pins formed integrally with and extendng from an edge of the sheet metal strip may be substituted for the wires of a conventional charging device with certain other structural modifications necessary for functional implementation as shown, for example, in U.S. Pat. No. 3,691,373 to Compton or U.S. Pat. No. 4,592,713 to Gundlach et al. In this arrangement, a corona is generated at the pin tips to impart the requisite charge onto the photoconductive surface. This coronode arrangement (hereinafter referred to as a pin array coronode) has significant structural and operational advantages over wire-type coronodes. A significant advantage of the device is reduced production of ozone, which is believed to be proportionally related to nitrogen oxide species production. The sheet metal coronodes have comparatively high structural strength in comparison to wire devices. It is particularly important that coronodes in a reproduction machine be resistant to breakage when subjected to excessive vibration or rough handling, such as occur during coronode cleaning. Such characteristics enhance field maintenance of reproduction devices incorporating breakage resistant coronode structures by reducing potential damage to the photoreceptor due to broken corotron wires, and possible electrical accidents from the dangling high voltage wire. Perhaps most importantly, periods of inoperativeness and the expense of repair are substantially avoided by the use of such structures. Additionally, devices such as dicorotrons require a high voltage alternating current power supply which requires expensive rectifying and transformer circuitry to alter the current to be satisfactory for use. By contrast, pin coronode devices may be driven by direct current voltage source substantially reducing the cost of rectifying and transformer circuitry in the power supply. It is also a feature of the pin array coronode that it produces a highly directionalized corona wind in comparison with wire-type coronodes. By contrast, wire-type coronodes have a significantly less directed corona wind directed radially about the corona producing surface of the wire.
When used to charge a photoreceptor, the pin array coronode finds particularly advantageous use in a scorotron arrangement combining the advantages associated with the use of pin array coronodes with the advantageous aspects of scorotron charging. Photoreceptor deletion problems associated with scorotron charging, however, have been enhanced in part due to the enclosure of the scorotron charging elements with the screen preventing substantial passage of air through the device. Additionally, it is common practice to prevent the flow of air through the scorotron members to prevent contamination thereof from dust or toner particles, which build up along surfaces of the scorotron member thereby changing electric characteristics of the device, causing current leakage from the corona device to the screen, and charging non-uniformities.